Diamond is an allotrope of carbon exhibiting a crystallographic network comprising exclusively of covalently bonded, aliphatic sp.sup.3 hybridized carbon atoms arranged tetrahedrally with a uniform distance of 1.545 .ANG. between atoms. Diamond is extremely hard having a high Mohs hardness of 10. It exhibits four times the thermal conductivity of copper and it is electrically insulating. As a result it is an ideal heat sink or a circuit substrate for semiconductor devices.
Diamond can be grown as an equilibrium phase at high pressures or it can be grown under metastable conditions at low pressures. The present invention is directed to growing diamonds at low pressures from hydrocarbon gases in the presence of atomic hydrogen. Many methods have been disclosed for growing diamonds metastably and generally these methods differ from each other by the way in which atomic hydrogen, a key reactant, is generated and transported within the system.
One class of methods developed in recent years for producing diamonds comprises chemical vapor deposition (hereinafter sometimes "CVD") methods. For a general summary of various diamond deposition methods including CVD methods, reference is made to Chemical & Engineering News, 67(20), 24-39 (May 15, 1989), incorporated herein by reference. In the CVD methods, a mixture of hydrogen and a gaseous carbon compound such as methane is activated and contacted with a substrate to produce diamond film on the substrate. The hydrogen gas is dissociated into atomic hydrogen and then reacted with the carbon compound to form condensable carbon radicals including elemental carbon. The carbon radicals are then deposited on a substrate to form a diamond. Some of the CVD diamond coating methods, hereinafter referred to as "filament" methods, employ one or more resistance heating units including heated wires or filaments, typically at temperatures of at least 2000.degree. C., to provide the high thermal activation temperatures necessary to bring about these conversions.
Various problems have been encountered in the filament methods of CVD diamond deposition. For example, it is difficult to create conditions under which the linear deposition rate of diamond is high enough to become commercially feasible for thick film applications. Numerous methods employing a horizontal configuration of the substrate(s) and helically wound filaments having been disclosed, but for the most part, the linear deposition rates afforded therefrom are relatively low.
The linear growth rate was improved by the use of a jet producing plasma torch that generates a plasma jet by means of a direct current (hereinafter "DC") arc, radio frequency (hereinafter "RF") energy or a microwave energy. The plasma jet produced by the plasma torch is hot enough to produce gases in their elemental form.
Even though the use of plasma jets improves diamond growth, the aforementioned plasma jet methods are still plagued by their low efficiency and their low substrate area coverage, i.e. the substrate coverage for a conventional DC are plasma torch is about one square centimeter. However, most of the commercial applications utilizing a thick free-standing diamond film, call for large film areas of over about 10 square centimeters. Thus, for the plasma jet process to be of commercial interest such a process should be able to produce thick (for example about 200 micrometers) free-standing diamond films having significantly large areas while still maintaining high diamond deposition rates.
An attempt was made to address some of the aforementioned problems by using multiple plasma jets to cover larger areas of the substrate. However, it is difficult to achieve a diamond film of uniform thickness by using such a method. The problem of a lack of a uniform diamond film thickness was partially addressed by using closely spaced multiple plasma jets, separated at about 2 millimeters. However, the closely spaced plasma jets severely reduce the life of a DC arc torch anode and it is extremely difficult to balance the power and the gas feeds to various plasma torches.
Another approach was to scan the plasma jet, produced by the plasma torch, over a larger area of the substrate to produce large area, thick (free-standing) diamond films. However, due to temperature modulation produced by such a scan, the diamond film produced therefrom has a tendency to crack.